Hydraulic units of the aforementioned type are either prefilled with brake fluid by the brake manufacturer or made available in the unfilled state to the vehicle producer. Because, in the case of motor vehicle brake systems operating on the recirculation principle, the wheel outlet valves lead into so-called secondary circuits which contain the entire pump suction conduit with the low-pressure accumulator, the pressure-retaining valve and the change-over valve, special venting and filling measures are needed in order also to protect the region of the secondary circuit separated by electrically and hydraulically switchable valves from air inclusions in an operatively safe manner and to fill same with brake fluid.
In this case it is advantageous for the vehicle producer if the process of filling the entire brake system is restricted solely to the primary circuit, that is, to the hydraulic section between the braking pressure generator and the pressure medium connection opened in the direction of the wheel brake. For this reason the essential functional components combined to form a hydraulic unit are often made available to the vehicle producer in a prefilled state by the brake system manufacturer.
A suitable approach to solving this problem is known from the patent EP 0 950 004 B1, which is incorporated by reference. In this patent it is proposed to provide a prefilling location between the pressure-retaining valve and the outlet valves in the pump suction conduit, via which prefilling location evacuation and prefilling of the entire secondary circuit is effected.